This invention relates to plasma channels, and to plasma-addressed liquid crystal display panels commonly referred to as "PALC" display devices using such channels. These devices comprise, typically, a sandwich of: a first substrate having deposited on it parallel transparent column electrodes, commonly referred to as "ITO" columns or electrodes since indium-tin oxides are typically used, on which is deposited a color filter layer; a second substrate comprising parallel sealed plasma channels corresponding to rows of the display crossing all of the ITO columns and each of which is filled with a low pressure ionizable gas, such as helium, neon and/or argon, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin transparent dielectric sheet; and a liquid crystal (LC) material located between the substrates. The structure behaves like an active matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of the backlight or incident light to each LC pixel is a function of the stored voltage across the pixel. A more detailed description is unnecessary because the construction, fabrication, and operation of such PALC devices have been described in detail in the following U.S. patents and publication, the contents of which are hereby incorporated by reference: U.S. Pat. Nos. 4,896,149; 5,077,553; 5,272,472; 5,276,384; and Buzak et al., "A 16-Inch Full Color Plasma Addressed Liquid Crystal Display", Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.
A cross-section of the PALC display described in the 1993 SID Digest is shown in FIG. 2. The method described in the referenced publication for making the plasma channels is to chemically etch a flat glass substrate to form parallel semi-cylindrically shaped recesses defined by spaced ridges or mesas and to bond on top of the mesas a thin dielectric cover sheet having a thickness in the range of about 30-50 .mu.m.
In order to make the cover sheet of the plasma channel part more rigid, U.S. Pat. No. 5,214,521 proposed a construction in which the electrodes are deposited on a flat bottom plate while the top plate is etched back to form channels shaped as hemi-cylinders--the inverse of that shown in FIG. 2--in such a way that the remaining glass at the top of the channels is thin enough to allow addressing of the adjacent LC material. However, due to the circular curvature of the top plate, between the plasma discharge and the LC material, the glass thickness and thus the voltage drop over the LC material varies significantly for each pixel. In practical situations, this reduces the number of gray levels of the display.
U.S. Pat. No. 5,349,454 describes a construction wherein the electrodes are formed on a flat substrate, and on top of the electrodes are formed insulating ribs that constitute the lateral mesas or separating walls of each plasma channel, a flat thin dielectric sheet then being adhered to the tops of the ribs. This patent does not describe how the ribs are formed, but a commonly-assigned laid-open EPO application, No. 0 500 085 A2 describes a forming process as screen printing. This process has certain disadvantages. These disadvantages include: 1) high resolution may be difficult to obtain, and 2) uniform rib height may also be difficult to obtain.